1. Field of the Invention
The invention relates to an electro-dynamic loudspeaker comprising a chassis, a translatable body with a three-dimensional diaphragm extending within the chassis, the diaphragm comprising a front part and a back part, and an electromagnetic actuator for displacing said body with respect to the chassis along a translation axis extending from said front part to said back part of the diaphragm, said actuator comprising a stationary actuator element fixed to the chassis and a translatable actuator element connected to the translatable body proximate to the back part of the diaphragm, said actuator elements being capable of magnetically co-operating with each other over an air gap, while a first flexible connecting means is present proximate to the front part of the diaphragm for movably journaling the translatable body with respect to the chassis, and a second flexible connecting means is present proximate to the back part of the diaphragm for movably journaling the translatable body with respect to the chassis.
2. Description of the Related Art
A dynamic loudspeaker comprising a conical diaphragm, a loudspeaker housing with a loudspeaker basket and an electromagnetic driver is known from German Gebrauchsmuster DE-U 75 02 568. The driver is accommodated in the housing, outside the diaphragm, and has a driving coil arranged on a coil former secured to the diaphragm, as well as a magnetic system secured to the loudspeaker basket. The diaphragm, which is within the loudspeaker basket, is fixed on its front side to the loudspeaker basket via a flexible suspension rim, and on its rear side via a centering means. In the known loudspeaker, the driver is situated in an area outside the loudspeaker basket so that the loudspeaker has a relatively large axial dimension. This has the drawback that the depth of the housing in practical embodiments is relatively large with respect to the front dimensions of the loudspeaker. Consequently, the known loudspeaker occupies a relatively large space of the volume constituted by the housing, which leads to relatively high system resonances, and thus, to a relatively high lower limit of the frequency range associated with a given volume.
It is an object of the invention to provide a compact loudspeaker that can be subjected to a high load. According to the invention, this object is achieved with the loudspeaker which is characterized in that the air gap, which extends between the stationary actuator element and the translatable actuator element, is present outside the space enclosed and defined by the contours of the diaphragm and within an area between the first flexible connecting means and the second flexible connecting means.
Due to the characteristic feature described above, the loudspeaker according to the invention has only a limited axial dimension in spite of its three-dimensional diaphragm, so that it has a small build-in depth. Such a loudspeaker built in a housing therefore occupies a comparatively small volume, resulting in a low system resonance. The loudspeaker according to the invention is therefore eminently suitable for use in subwoofer systems in which compact, shallow housings are desired. The diaphragm used preferably has a conical central portion.
It is to be noted that European Patent Application No. EP 0806883, corresponding to U.S. Pat. No. 5,898,786, discloses a loudspeaker in which the electromagnetic drive of the diaphragm is situated within the space enclosed and defined by the contours of the diaphragm. In such a xe2x80x9cwaferxe2x80x9d loudspeaker, the available space within the diaphragm is, however, too small to build a sufficiently powerful magnetic system which complies with the high load requirements. Apart from the available space, the thermal load of the magnetic material of the magnetic system is also a limiting factor in this type of loudspeaker. Notably with modern magnetic material such as neodymium-iron-boron alloys, the maximum allowable temperatures are relatively low. For these reasons, a loudspeaker of this type is only suitable for loudspeaker systems having a relatively small power.
In a practical embodiment of the loudspeaker according to the invention, the stationary actuator element is at least largely, and preferably completely, present within a zone situated between the first flexible connecting means and the second flexible connecting means of the loudspeaker.
A preferred embodiment of the loudspeaker according to the invention is characterized in that the stationary actuator element comprises an annular permanent magnet, and the translatable actuator element comprises a magnet coil, also referred to as voice coil, the magnet coil being situated in a field surrounding the magnet, i.e., the magnetic field of the magnet.
In this embodiment, annular magnet coils having large coil diameters are possible. Such drive coils have large cooling faces and offer ample possibilities of ventilation so that they can be subjected to high loads. Consequently, the coil temperature remains relatively low during use, also in the case of high loads, and only a limited transfer of heat to the other parts, such as the permanent magnet of the actuator, takes place. This embodiment, therefore, has a favorable thermal household. It has been found that the temperature of the coil and the magnet remains within acceptable limits, also at higher powers. Modern magnetic materials, such as neodymium-iron-boron alloys, can, therefore, be used without any problem.
It is to be noted that International Patent Application No. WO-A 97/32451, corresponding to U.K. Patent Application No. GB 2,325,373A, discloses a loudspeaker unit which is provided with a damper fixed to a diaphragm. A coil former, with a coil extending in a magnetic gap surrounding a plate on a magnet, is fixed to the diaphragm, this gap extending outside the area bounded by the suspensions of the diaphragm. The magnet, which is situated at a maximum distance from the diaphragm, is fixed to a bottom part of the unit.
In a practical embodiment of the loudspeaker according to the invention, the first flexible connecting means has a flexible structure with an undulation or wrinkle, this first flexible connecting means being fixed to the chassis at one end and to the translatable body at the other end. In principle, the second flexible connecting means may be formed similarly.
To allow relatively large and accurately defined axial displacements of the translatable body with respect to the chassis, a preferred embodiment of the loudspeaker according to the invention is characterized in that the second flexible connecting means comprises a set of at least three leaf spring elements positioned around the translation axis, each leaf spring element comprising two leaf springs arranged in a slanting position with respect to a plane oriented transversely to the translation axis, the leaf springs of each leaf spring element being each provided with two peripheral parts each extending parallel to said plane, while one of the leaf springs is fixed to the chassis proximate to one of its peripheral parts and the other of its peripheral parts is fixed to the translatable body proximate to one of its peripheral parts, the leaf springs of each leaf spring element being interconnected proximate to their other peripheral parts. This embodiment of the second flexible connecting means allows a large axial displacement of the diaphragm, and is, therefore, suitable for applications where loudspeakers having a high sound-radiating power are desired.
The flexible connecting device used as the second flexible connecting means described hereinbefore, is essentially without friction and guarantees an accurate alignment of the translatable body and, hence, of the diaphragm with respect to the chassis. Embodiments of this connecting device are described and elucidated in European Patent Application No. EP-A 1036485, corresponding to U.S. Pat. No. 6,385,327, herein incorporated by reference. In this respect, it is only noted that the leaf springs of each leaf spring element are preferably connected or fixed to each other in a mutual undeformable manner at their adjacent peripheral parts. Furthermore, one leaf spring of each leaf spring element is preferably secured in or fixed to the translatable body in an undeformable manner and the other leaf spring is secured in or fixed to the chassis in an undeformable manner by means of one of its peripheral parts.
If desired, the loudspeaker according to the invention may be used as a band-pass loudspeaker for a limited frequency range. This may be achieved in an electrical manner. Notably in low-frequency loudspeakers, the electrical measures to be taken are, however, relatively expensive. A mechanical solution which forms part of the loudspeaker is less expensive and is, therefore, preferred. A suitable embodiment in this context is characterized in that the translatable actuator element is fixed via a coupling member to the translatable body with a defined bending stiffness and internal damping. The concrete values of the bending stiffness and the damping are determined as a function of the desired frequency range and the characteristics of the loudspeaker. A simulation program may be used for this purpose.
A practical embodiment of the band-pass loudspeaker according to the invention is characterized in that the translatable actuator element comprises a magnet coil arranged on a coil support, the coupling member connecting the coil support to a central portion of the translatable body.
The invention also relates to a loudspeaker unit comprising the electro-dynamic loudspeaker according to the invention and also comprising a housing accommodating the loudspeaker. The loudspeaker unit may form part of a compact subwoofer system, in which the loudspeaker or loudspeakers can be subjected to a high load. The loudspeaker according to the invention occupies only a limited space in the housing.
The loudspeaker and loudspeaker unit according to the invention may be successfully used in loudspeaker systems having very flat shapes, possibly combined with panel loudspeakers. Furthermore, applications in which only a limited mounting depth is available are suitable. Particular examples are automotive uses, such as, for example, mounting in a car door, in a luggage shelf or below a car seat. High powers of the order of, for example, about 150 watts are possible.
These and other aspects of the invention are apparent from and elucidated with reference to the embodiments described hereinafter.